1. Field of the Invention
The present invention relates to systems and methods for casting thin films, and in particular to a casting head using a variably displaceable cavity surface.
2. Description of the Related Art
Precision tape casting machines are designed to produce thin sheets and layered products to precise tolerances. Such casting machines typically use a casting head having a cavity filled with a fluid casting material such as an inorganic powder loaded polymer, or an aqueous slurry. The bottom of the cavity (and hence, the fluid material) is exposed to a carrier film, which is drawn across the casting head. As the carrier film passes the casting head, the fluid material is deposited on the carrier film. The fluid material is then dried, and if necessary, removed from the carrier film. The casting machine includes a rigid structure with a solid, level casting platform that uniformly supports the moving carrier surface.
For production of a high-quality film, the conditioned casting material must be delivered consistently to the surface of the carrier media. This is accomplished by a precision gap provided by an adjustable doctor blade, which is positioned on a precision-ground casting surface certified to tolerances as tight as 0.5 mils. To assure that such machines produce a consistent film, the carrier speed is precisely controlled using a microprocessor-controlled digital drive with electronic and visual monitoring.
In producing high-quality film, it is also important that the casting material be applied to the carrier media with the same pressure. Unfortunately, as the casting process proceeds, the amount of casting material in the casting head is depleted, thus reducing the pressure exerted by the casting material at the carrier media. Since this phenomenon results in undesirable film variability, the casting material in the casting head is replenished.
While this process is effective, it requires the use of a large amount of expensive equipment operating at tight tolerances. Further, this process is inefficient for short casting runs where less than about 100 feet of film is required. Cleaning up the casting machine and disposing of excess casting material can be a lengthy process, requiring the disposal of potentially hazardous waste.
From the foregoing, it can be seen that there is a need for a casting head that does not require the use of expensive equipment. It can further be seen that there is a need for a casting head suitable for application to short casting runs, without consuming excessive casting material or requiring expensive cleanup procedures. The present invention satisfies that need.
To address the requirements described above, the present invention discloses a casting head having a variably displaceable cavity surface, and a method for using the casting head to produce films.
The casting head comprises a cavity, at least partially formed by plurality of surfaces, including a carrier surface adjacent the cavity, and a variably displaceable drive surface for controlling the pressure of a viscous casting material in a region proximate the carrier surface. In one embodiment of the invention, the casting head further comprises a doctor blade, disposed proximate the carrier surface, forming a gap therebetween for egress of the casting material. In another embodiment of the casting head, the variably displaceable drive surface is comprised of a piston. The piston is actuated by a driver-controlled motor according to a pre-determined displacement program, or according to the level of the casting material in the cavity.
The method comprises the steps of providing a casting material to a casting head cavity having an aperture in fluid communication with the carrier surface, displacing the carrier surface relative to the cavity to deposit the casting material on the carrier surface, and displacing the casting head cavity surface to control a pressure of the casting material in a region proximate to the carrier surface. In one embodiment, this is accomplished by way of a piston, which is displaced so as to maintain the level of casting material in the cavity.
As will be discussed herein, the casting head of the present invention produces significantly better films than prior art casting heads. Further, the casting head of the present invention is more efficient and provides much better film parameter control. The resulting films more precisely match production specification, and can be produced in less time and are more cost effective than films produced with the traditional casting head.
For virtually any size film casting, the constant pressure casting head of the present invention executes the run more quickly and with less wasted casting material. Because much less equipment is required for a casting run with this invention, the cleanup time is cut to a fraction of what it was with the traditional casting head setup and casting material waste is almost nonexistent. Further, the present invention yields more film for the amount of raw material used, thus providing a more reproducible process and provides higher quality castings than the traditional casting methods in use today.
The present invention is easily implemented and is effective for short, medium or any length casting runs. Whenever a new or different slurry loading or slurry composition is required, the present invention allows material waste to be reduced at a minimum and reductions in the costly, labor intensive cleanup process by effectively cutting the costly, labor intensive cleanup time of the complex traditional casting equipment in half. It allows scientists, engineers and technicians to experiment with different casting material mixtures, casting material viscosities and processing parameters at minimal cost to the customer. When a specific length of cast film is required, a complimenting volume of slurry can be tailored to it and our new casting head precisely calibrated for that volume. This can be implemented in less time and executed in significantly less time than the traditional method, saving the time and money. Even if used in production-oriented processes, the present invention allows the performance of sample runs to establish the proper parameters for longer film casting runs without wasting expensive ingredients or spending excessive time on setup and cleanup.